1. Field of the Invention
The present invention relates to a rocking member apparatus having a rocking member that is rockable about an axis. Specifically, the invention relates to an optical deflector in which a rocking member for deflecting incident light is rocked by utilizing electromagnetic force, and to an image display apparatus incorporating the optical deflector.
2. Related Background Art
Devices for deflecting/scanning light beams such as laser light (such devices are referred to as the “optical deflectors” in this specification) are currently used in optical equipment such as laser printers and bar-code readers. Conventional examples of optical deflectors to be incorporated in those equipment include polygon mirrors for changing the reflection direction of incident light by rotating a polygonal prism having mirror surfaces formed on its lateral faces, and galvano mirrors comprising a plane mirror that is rotationally vibrated by an electromagnetic actuator.
The following are required in the conventional mirrors described above. That is, the polygon mirrors require an electromagnetic motor for rotating the mirrors, and the galvano mirrors require a mechanical drive coil and a large yoke for generating a magnetic field. Further, the requirement of obtaining a high output torque limits miniaturization of those mechanical elements. Also, the spaces required for vertical stacking of the respective components and other factors contribute to limit overall size reduction of the apparatus for performing optical deflection.
It is a fundamental knowledge in electromagnetics that a torque is generated around a magnet placed under uniform magnetic fields. Assuming that m represents a magnetic moment (vector) of the magnet and H represents a magnetic field, a torque T (vector) is expressed as follows:T=m×H  (1)
A scanning mirror driving apparatus (refer to Japanese Patent Application Laid-Open No. 6-82711, for example) is an example of optical deflectors utilizing the above knowledge. FIGS. 16 to 18 illustrate such a scanning mirror driving apparatus as an optical deflector disclosed in Japanese Patent Application Laid-Open No. 6-82711. FIG. 16 is a perspective view showing Example 1 of the invention disclosed in the same document. The optical deflector includes a magnetism generating portion 1106 having a coil 1107 wound around a coil frame 1108, and a mirror portion 1101. The mirror portion 1101 includes a glass plate 1102, a mirror surface 1103, and a thin-film permanent magnet 1104, and is supported by a pair of supporting members 1105 so as to be rotatable about a rotation axis 1109 relative to a main body (not shown) of the apparatus. The thin-film permanent magnet 1104 is previously magnetized so as to have opposite polarities across the rotation axis 1109. The magnetism generating portion 1106 is disposed on the side of the permanent magnet 1104 of the mirror portion 1101, at a predetermined distance from the mirror portion 1101.
By supplying an electric current to the coil 1107, a magnetic field is generated from the magnetism generating portion 1106 whereby the mirror portion 1101 is rotated due to a torque exerted thereon in accordance with the aforementioned formula (1). Irradiating light onto the mirror surface 1103 at this time allows the light to be deflected/scanned.
FIG. 17 shows Example 2 of the invention disclosed in Patent Document 1. In the optical deflector disclosed therein, the magnetism generating portion 1106 having the coil 1107 wound around the coil frame 1108 is replaced by a magnetism generating portion 1116 having a coil 1117 and an iron core 1119. Other structural portions of the optical deflector are the same as those of Example 1 of the invention disclosed in the same document.
FIG. 18 illustrates Example 3 of the invention disclosed in Patent Document 1. The optical deflector disclosed therein is composed of a magnetism generating portion 1126 having a coil 1127 wound around a coil frame 1128, and a mirror portion 1121. The mirror portion 1121 includes a glass plate 1122, a mirror surface 1123, and a thin-film permanent magnet 1124, and is supported by a pair of supporting members 1125 so as to be rotatable about a rotation axis 1129 relative to the magnetism generating portion 1126. The thin-film permanent magnet 1124 is previously magnetized so as to have opposite polarities across the rotation axis 1129. The magnetism generating portion 1126 is coupled to an end portion of each of the supporting members 1125 and is arranged in the outer periphery of the mirror portion 1121.
The optical deflector disclosed in the cited Patent Document 1 is designed for use in a laser displacement sensor or the like for scanning and detecting minute surface irregularities or scars by utilizing interference of laser beams.
The present inventors have found that, with the optical deflector disclosed in the above document, it is difficult to sufficiently enlarge the deflection angle in the mirror portion, to cause high-speed rotation of the mirror portion, or to reduce power consumption.
In the optical deflector of the above document, in order to increase the rotational speed or to attain a large deflection angle (i.e. to increase the rotation angle of the mirror portion), it is required to increase the magnitude of a torque acting on the mirror portion. Further, for thus increasing the magnitude of the torque, it is required to increase the magnitude of a magnetic field at the mirror position. In that case, it is required that the magnetism generating portion be arranged in close proximity to the mirror portion or that a larger current be made to flow in the coil.
In the optical deflector of the above document which has a construction shown in FIG. 16, the coil is disposed underneath the mirror portion in close proximity thereto. In this case, bringing the magnetism generating portion into proximity of the mirror portion will result in unwanted contact between the mirror and the coil. Therefore, the mirror cannot make rocking movement over a wide angle.
Another problem with the above construction is that, as seen in the vertical direction of the coil (vertical direction with respect to the mirror surface), the lower part side of the coil is situated apart from the mirror portion and thus its contribution toward magnetic field generation is small. Such a construction is disadvantageous to generation of a torque, since the strength of a magnetic field is inversely proportional to the square of the distance between the mirror and the coil that surrounds it. In order to obtain a large torque with such a construction, it is necessary to cause a larger current to flow in the coil, which results in increased power consumption. Further, the vertical winding arrangement of the coil makes it difficult to achieve vertical miniaturization of the apparatus.
Further, the apparatus according to Example 2 of the invention disclosed in the above document is of a type in which the iron core 1118 is used to increase the strength of the magnetic field generated from an upper end portion of the coil. This apparatus also suffers from the problems in that vertical miniaturization cannot be attained and that the lower end portion of the coil makes only small contribution toward generation of the torque. Further, the iron core 1118 needs to have a large sectional area relative to the mirror 1111 opposing the iron core 1118.
Considering the large vertical thickness of the coil 1117, the above construction results in increased size of the apparatus, as well as increased weight due to the use of an iron core in order to increase the magnetic field strength.
The apparatus according to Example 3 of the invention disclosed in the above document, which is designed to achieve vertical miniaturization and lightweightness by winding the coil 1127 so as to surround the mirror 1121, is of a type in which no iron core is used. Also with this construction of the apparatus, upper and lower end portions of the vertically wound coil make only small contributions toward magnetic field generation.
Therefore, a large amount of power is required in order to incline the mirror 1121 by large angles.
Further, the present inventors have presumed that, in order to increase the scanning speed (rocking speed) of the mirror, it is necessary to increase hardness of the spring of the drive shaft. In this regard, however, the present inventors have found that increasing the hardness of the spring in the aforementioned optical deflector results in a further increase in power consumption, since, in the first place, the optical deflector has a large room for improvement in terms of its power consumption.